toString() should work on all literal types

[piotrszul]

`toString()' fails for Boolean and Quantity collections.
Also it should produce {} for not literal types.

(Literal exclusions)

Details

java.lang.RuntimeException: Error invoking method-defined function

• attrtrue.toString() [** bool to string]

(FHIR R4 Exclusions)

Details

java.lang.RuntimeException: Error invoking method-defined function

• true.toString() [** testBooleanLiteralToString]

Details

java.lang.RuntimeException: Error invoking method-defined function

• 1 'wk'.toString() [** testQuantityLiteralWkToString]

[johngrimes]

I think toString is actually a new feature that has not been implemented completely, and we should go ahead and complete the implementation.